This invention relates to articles comprising graphite and more particularly to high tensile modulus compositions comprising highly oriented graphite and methods for the production of such compositions.
Carbon structures are widely used for applications where high temperatures will be encountered and where heat dissipation is important such as, for example, in high energy brake pads and in consumer electronic devices as electronic heat sinks. While a good balance of mechanical properties continues to be important in such demanding applications, high thermal conductivity and good dimensional stability have become particularly important considerations. The thermal conductivity and dimensional stability of solid carbon depends largely on its structure. Characteristically, these properties improve as the crystallinity and density of the carbon increases. Solid amorphous carbon may typically have a density near 1.2 g/cc and a thermal conductivity as low as 100 w/m-xc2x0K, while single crystal graphite has a density of about 2.26 g/cc, a thermal conductivity near 1800 w/m-xc2x0Kxe2x80x94considerably greater than the conductivity of copperxe2x80x94and, unlike metals, a negative coefficient of thermal expansion. These characteristics are highly desired by users of carbon articles, and the art has expended considerable effort seeking methods for producing carbon structures with such high densities reproducibly and with good control.
Highly ordered pyrolytic graphites having densities near 2.2 g/cc and good thermal conductivity have been produced by vapor deposition of carbon. Highly oriented pyrolytic graphite (HOPG) may have a thermal conductivity on the order of 800 w/m-xc2x0K. However, the HOPG materials are extremely fragile, too brittle even for measurement of mechanical properties such as tensile strength, and are extremely costly to produce. The process is extremely costly, and is capable only of producing very small, extremely fragile, wafer-like articles on the order of about one-half to one inch square. HOPG materials are thus severely limited in their application and have not found wide acceptance.
The bulk graphites widely used commercially for fabricating articles such as crucibles, electrodes and the like are largely amorphous and relatively low in density, and lack the high thermal conductivity of crystal graphite. To the extent particular bulk graphites may be crystalline, the crystalline component will comprise large, randomly-oriented graphitic crystallites, generally greater in size than about 30 to 50 microns, embedded in a substantially amorphous carbon phase. These lower-density bulk graphite articles will generally exhibit only a fraction of the bulk thermal conductivity that characterizes highly organized crystalline graphite. The degree of crystallinity in bulk graphite structures may be altered by optimizing a number of process factors including annealing and by the nature of the pitch employed. Mesophase or liquid crystal pitch may be readily transformed thermally into a more crystalline bulk graphite; however, bulk mesophase pitch is generally not oriented and, when processed in bulk into crystalline graphite, the crystallites also lack orientation. Although the density of these bulk graphite articles thus may be higher than for other forms of bulk carbon, the bulk thermal conductivity is considerably below that of crystal graphite.
Bulk graphites also lack the mechanical strength needed for more demanding thermal applications. Adding carbon or graphite fiber reinforcement directly to bulk pitch prior to thermal processing may afford modest improvement in mechanical properties. As with most composite materials, control of fiber reinforcement configuration through use of carbon fiber fabric or other structured preforms may permit further improvement in properties. Infiltrating a carbon fiber preform with pitch or vapor-deposited pyrolytic carbon to serve as binder and matrix, then carbonizing and graphitizing, will provide composites with improved the mechanical properties. However, even with use of pressure consolidation, the articles will generally have densities below about 1.5 g/cc and correspondingly low thermal conductivities, generally below about 500 w/m-xc2x0K. In addition, infiltrating a preform with pitch or with a vapor-deposited carbon is difficult, time-consuming and expensive.
One widely-used method used for producing reinforced carbon articles has been to coat sheets of graphite cloth with a suitable binder, stack the sheets and heat the structure to carbonize the binder. In U.S. Pat. No. 4,178,413 a process is disclosed whereby a woven carbon fiber structure is formed, for example, from carbonized cloth of rayon or polyacrylonitrile (PAN), infiltrated with a vapor-deposited pyrolytic carbon to bond the substrate fiber, then impregnated with a carbonizable filler, cured under pressure and finally carbonized. Five to ten cycles of impregnating and carbonizing are necessary to produce a carbon article having a density suitable for carbon brake use, disclosed therein as in the range of 1.5 to 1.85 g/cc. Such processes are extremely difficult to carry out without introducing variation in density, void formation and cracking.
An alternative to pitch infiltration processes, disclosed in U.S. Pat. No. 4,849,200, employs a preform constructed from an intimate combination of pitch fiber and a pitch-based carbon fiber reinforcement. When placed under an applied pressure of at least 10 kg/cm2 and fully carbonized thermally, the pitch fiber component apparently melts and flows to supply the matrix component of the composite cementing the reinforcing fiber. The volume fraction of the fiber reinforcement in the resulting composite will generally be less than about 70 volume %, and the bulk density of the composites is seen to be generally less than about 1.7 g/cc.
Binderless processes involving thermal processing of a carbonized pitch fiber bundle are also known. These processes are ordinarily carried out using extreme pressures, externally-applied, to compact the structure and force the carbonized pitch to flow and cement the fiber bundle. For example, U.S. Pat. No. 4,032,607 discloses forming staple lengths of fiber by spinning a carbonaceous pitch, preferably by blow spinning, and depositing the fiber on a screen to form a web. The web is then heated in air to oxidize the fiber surfaces to an oxygen level of 1 to about 6 wt. %, which generally is sufficient to stabilize the fiber mat or felt without completely thermosetting the fiber and rendering the fiber infusible. Further heating in an inert atmosphere under pressure causes unoxidized pitch to flow and exude through defects in the fiber, providing a pitch matrix to bind the fiber. Carbonizing the structure provides a low-density carbon composite with a high degree of porosity.
As disclosed in U.S. Pat. No. 4,350,672, a preform made from acrylic fiber oxidized to a level of oxygen sufficient to render the fiber non-melting is first consolidated by applying heat and pressure and then carbonized and graphitized by heating in an inert gas atmosphere, providing a carbon body said to have a fibular microstructure and to be porous, the level of porosity ranging from 2% for very high consolidation pressures to greater than 70% when lower consolidation pressures are employed. The consolidation step is characterized as causing individual fibers to bond together, with heat distortion flow increasing the contact area of the fibers and promoting bonding between contiguous fibers. These structures may be more appropriately described as porous, sintered, fibrous bodies.
In U.S. Pat. No. 4,777,093, there is disclosed a process wherein pre-oxidized PAN fiber having an oxygen content in the range of 9 to 14 wt. % is first subjected to a series of forming operations to produce lengths of densified tow having fiber density of up to about 75 to 80%, then infused with water or other suitable plasticiser to swell the fiber and leach low polymer from the fiber interior. The tow structure is then encapsulated with low temperature metal alloy and subjected to hot isostatic pressing at pressures as great as 15,000 psi. After first melting and removing the metal alloy, the resulting carbon body may then be graphitized by heating under inert atmosphere at temperatures as great as 2500xc2x0-3200xc2x0 C. to have a density as great as 2.1 g/cc and a thermal conductivity in the range of 350-400w/mxc2x0K. Although the conductivity is generally higher than for graphitized PAN fiber, it is inadequate for most thermal applications. In addition, the rigidity of the graphitized structure is undesirably low, with modulus values for the graphitized body generally below 50xc3x97106 psi.
It is apparent that the processes heretofore available in the ant for producing high density carbon articles a generally unsatisfactory. Most are very expensive to practice, requiring specialized equipment capable of achieving high pressures and temperatures, and may require inordinately long times, often on the order of months to complete, further adding to cost. Highly ordered graphites are generally brittle, while reinforced structures lack the necessary thermal properties. Generally, the better reinforced carbon articles known in the art have thermal conductivities less than about 300 w/m-xc2x0K, while most bulk graphites exhibit lower thermal conductivity , even as low as 50 w/m-xc2x0K. Such carbon articles also generally lack the highly desirable negative coefficient of thermal expansion characteristic of crystal graphite, and many also are deficient in mechanical properties, particularly tensile strength and rigidity.
The demand for carbon articles that combine high thermal conductivity, a desirable balance of mechanical properties including good tensile properties and high modulus, greater than 70xc3x97106 psi, and a negative coefficient of thermal expansion continues to grow. A high degree of dimensional stability, rigidity at high temperatures and excellent thermal conductivity are, in combination, increasingly important design requirements, and are particularly desired for applications where weight reduction is important, for example in consumer goods including electronic devices. Carbon structures exhibiting these highly desired properties and preferably with a high degree of anisotropy, that is, directed as desired along an axis of the structure, and a method for making such structures would be particularly useful and desirable for constructing heat sinks for electronic devices and in the design of brake materials for very high friction loads.
The carbon articles of this invention comprise a self-reinforcing, pitch-based carbon and possess an unique combination of excellent thermal conductivity and outstanding mechanical properties. Carbon articles according to this invention may exhibit, anisotropically, a thermal conductivity greater than 600 w/m-xc2x0K, a tensile strength greater than 10,000 psi and a modulus greater than 75xc3x97106 psi, together with a negative coefficient of expansion. The degree of anisotropy may be selectively controlled by modifying the overall degree of orientation, providing substantial capability for controlling thermal properties and dimensional stability characteristics when designing carbon structures for particular end uses without a concomitant sacrifice in strength. The carbon articles of this invention thus represent a considerable improvement over bulk graphite and reinforced carbon composites heretofore widely employed in the art.
Pitch-based carbon articles according to this invention are formed by carbonizing and graphitizing structures comprising mesophase pitch fiber treated with a suitable liquid oxidizer.
The pitch-based continuous fiber suitable for use in constructing the preforms according to the invention are produced by spinning a high purity mesophase pitch. Among the pitches useful for these purposes are high purity mesophase pitches obtained from petroleum hydrocarbon or coal tar sources. Methods for preparing a suitable pitch include those disclosed in U.S. Pat. Nos. 3,974,264, 4,026,788, and 4,209,500, and any of these methods as well as the variety of solvent-based and pitch fractionation processes known in the art may be employed for these purposes. Several methods are known and used in the art to characterize the mesophase component of pitch, including solubility in particular solvents and degree of optical anisotropy. The mesophase pitch useful in the practice of this invention preferably comprises greater than 90 wt % mesophase, and preferably will be a substantially 100 wt. % mesophase pitch, as defined and described by the terminology and methods disclosed by S. Chwastiak et al in Carbon 19, 357-363 (1981). Suitable pitches also include pitches synthesized from other chemical substrates by a variety of well-known processes. For the purposes of this invention, the pitch will be thoroughly filtered to remove infusible particulate matter and other contaminants that may contribute to the formation of defects and flaws in the fiber.
The mesophase pitch is formed into filaments by being spun from the melt using conventional methods, and the filaments are gathered to form a yarn or tow. For the purposes of this discussion, fiber is intended to be understood as including all collected continuous multifilament structures or bundles, including yarn, tow, strand or the like. In general, the spinning is conducted by forcing the molten pitch through a spinnerette while maintaining the pitch at a temperature well above the softening temperature. However, the temperatures useful for spinning generally lie in a narrow range and will vary, depending in part upon the viscosity and other physical properties of the particular pitch being spun. Those skilled in the art of melt-spinning will recognize that even though the pitch may be in a molten state, it may be too viscous or may have insufficient strength in the melt to form a filament and may even decompose or de-volatilize to form voids and other flaws when the pitch temperature is outside the temperature range useful for spinning that pitch. Thus it has long been a necessary and standard practice in the art to conduct initial tests to establish the temperature range that will be effective for melt spinning the particular pitch being employed. For the purposes of this invention, the pitch will preferably be spun at or near the highest temperature within in the effective range of spinning temperatures at which the pitch may be spun.
It is desirable to obtain very high degrees of orientation of the filamentary mesophase domains within each filament. While not wishing to be bound by any particular theory of operation, it appears the degree of crystallization that may take place during the subsequent thermal carbonization steps to form microcrystalline graphite, as well as the size of the crystallites that may form, is related to size of the mesophase domains in the filaments of the pitch fiber and the degree of orientation of the mesophase domains. Pitch fibers having large, well-oriented mesophase domains tend to form fibers comprising larger, more compact filamentary graphitic microcrystals upon being carbonized. The size, particularly the length of the filamentary mesophase domains as determined by Lc, and the degree of domain orientation appear in turn to be determined at least in part by the conditions employed for spinning the pitch fiber as well as by the nature of the pitch.
It is well-known that pitch tends to polymerize when heated, and to coke, particularly when exposed to an oxidizing environment while hot. Polymerization may in turn increase the melt viscosity of the pitch, making spinning difficult or impossible, while coking of the pitch forms infusible particles that contribute to flaws in the fiber and may block the spinnerette. The spinning process will therefore preferably be conducted using melting and heating operations designed and optimized to protect the molten pitch from exposure to air or other oxidizing conditions during the spinning operations, and to minimize the time the pitch is exposed to elevated temperatures.
Pitch fiber as spun is extremely soft and fragile and is thermoplastic, the filaments within the yarn readily undergoing creep and flow and becoming fused. Treatment of the pitch fiber with a liquid oxidizer such as aqueous nitric acid serves to modify the filament surfaces and to supply some lubrication as well, providing sufficient damage tolerance to permit the fiber to be handled and fabricated. The concentration of nitric acid needed will depend in part upon the length of time the pitch will be in contact with the acid A concentration of as low as 5 wt. % may be found effective for some purposes, particularly with extended exposure times, and substantially higher concentrations, as high as 35 wt % and more, may also be found useful. However, treatment using high concentrations of nitric acid is more difficult to control, and may render the pitch fiber thoroughly intractable and unsuited for use in the practice of this invention. Careful attention to and control of such process parameters as length of treatment, temperature and the like is therefore necessary to minimize the potential for infusibilizing the fiber. Lower concentrations, for example in the range of from about 5 to about 20 wt. %, preferably from about 10 to about 15 wt. %, will thus be preferred, while high concentrations of nitric acid, particularly above about 20 wt. %, will be less preferred. It will also be known to those skilled in the chemical arts that treatment of carbonaceous materials with highly concentrated oxidizers such as nitric acid caries the risk of producing a rapid, exothermic and possibly sudden or even explosive decomposition of the oxidized materials; hence, excessive concentrations of nitric acid are to be avoided. The liquid oxidizer may be applied to the pitch filaments as they exit the spinnerette prior to being gathered to form pitch fiber or yarn, or after being collected. A variety of methods for applying liquids to continues fiber are known, including dipping, spraying, misting and the like, as will be readily apparent to those skilled in the art. A rotating kiss wheel, commonly employed for the application of sizing to fibers, may also be conveniently used for this purpose.
Pitch fiber commercially employed for carbon fiber production is ordinarily rendered infusible by being treated in a thermosetting operation such as by heating in an oxidizing gas atmosphere at a temperature in the range of from 200xc2x0 to 400xc2x0 C., thus becoming able to withstand considerable working, abrasive contact and thermal exposure during the carbonizing and graphitizing operations without loss of fiber character. Treatment with nitric acid has also been used in producing carbon fiber. These oxidation processes often employ a particulate material such as carbon black or colloidal graphite to separate the pitch filaments and thereby reduce sticking, and surfactants may also be employed to maintain such particles as a uniform dispersion in the aqueous acid composition and aid the flow of the oxidizing composition over the fibers.
To be suitable for the purpose of providing carbon articles according to this invention it is essential that the pitch fiber remain thermoplastic and fusible; that is, the pitch fiber must be capable of undergoing thermoplastic flow and becoming fused when heated. Pitch fiber made infusible by oxidation using any of the thermosetting processes commonly employed in the carbon fiber art will thus generally be unsuited for use in the practice of this invention. It will also be apparent that the particulate materials and surfactants employed in such processes are intended to impede fusion and hence should generally be avoided when treating fiber intended for use in producing carbon articles according to the invention. Thermoset or otherwise infusible pitch fiber will not become fully fused when carbonized and graphitized according to the processes of the invention. The resulting carbonized structure will then comprise fiber poorly-bonded together at the interfaces and will thus be discontinuous and void-filled, low in strength and in bulk density.
The acid-treated fiber may be fed from the spinning operation directly to a fabricating operation for producing a preform as, for example, by weaving or filament winding, or the fiber may be accumulated by winding on a spool or bobbin, placed in a protective wrap, then stored for later fabrication. The wet fiber will contain considerable amounts, even as much as 50 wt. % aqueous acid, preferably from 30 to 45 wt. %, more preferably from 34 to 38 wt. % aqueous acid.
The acid-treated fiber will be fabricated into a preform structure, then carbonized and graphitized. The preform may be formed directly from the fiber while still wet with nitric acid, then directly subjected to the heat treating step. Generally, however, it is preferred to store the acid-wet preform in a protective bag or container for a period of from several hours up to about 14 days, thus providing an aging step whereby the acid may impart an adequate level of stabilization to the fiber surfaces. Although not required for the practice of the invention, such an aging step may be found effective for achieving optimum properties in the final carbon structure.
In the simplest embodiment, multifilament tow or unidirectional pitch fiber tape may be used directly as a preform or, more preferably, a plurality of lengths of tape, yarn or tow may be placed together in parallel relationship to provide a block or brick preform Alternatively, a suitable preform structure may be formed by conventional filament winding techniques using a continuous pitch fiber in the form of yarn or tow. In a particularly useful embodiment the pitch fiber may be wound on a bobbin or spool to form a cylinder. The cylinder is then sectioned by cutting longitudinally and the cut cylinder is opened to form a flat wafer or tablet comprising pitch fiber aligned substantially in the plane of the tablet The wafer may if desired be further cut or shaped prior to carrying out the thermal treatment steps. Carbonizing and graphitizing the preform as taught will provide a self-reinforced carbon plate.
In a further alternative embodiment, the cylinder obtained by winding the pitch fiber on a bobbin may be sectioned by slicing along planes perpendicular to the cylinder axis to provide a plurality of toroids or donut-like preforms having pitch fiber distributed circumferentially about the center of the toroid preform in the plane of the preform. It will be apparent that the wound cylinder may also be carbonized and graphitized to provide a cylindrical composite, which then may be sectioned or further trimmed and shaped to provide the desired carbon article. It will be understood that the bobbin employed for the winding operations may take a form other than cylindrical, and may if desired be faceted, thus providing further opportunity for controlling shape and fiber configuration in the final preform structure.
Any of the wide variety of methods well known in the art for fabricating carbon fiber may be adapted for use in the practice of this invention. For example, with suitable equipment, wet fiber tow may be formed into a uni-tape or even woven into a cloth or fabric, then formed into structure comprising one or a plurality of layers of such tape or fabric and finally carbonized and graphitized to provide the self-reinforced carbon article.
It will be recognized that the degree of fiber alignment in preform structures may be varied. For example, unidirectional acid-treated pitch fiber tape may be layered in a manner that will provide quasi-isotropic laminate structures. When obtained from a cylinder of wound tow, the fiber alignment in the preform will depend upon the winding angle used in placing the fiber on the bobbin, a low or zero winding angle giving a high degree of fiber alignment and greater winding angles serving to reduce alignment. This feature affords convenient control of fiber alignment in the preform, thereby permitting control of the level of property anisotropy in the resulting carbon article. For example, a xc2x145xc2x0 winding angle would provide a structure with quasi-isotropic properties in the fiber plane of the resulting composite, while a 0xc2x0 wind angle would provide a unidirectional structure having properties maximized along the fiber axis and in the fiber plane. A variety of filament winding techniques are widely used in the art for producing filament-wound structures, and these may also be adapted to provide preform structures from pitch fiber in a wide variety of wound shapes and with selectively determined fiber orientations for use in producing unique self-reinforced carbon articles.
Methods for providing preform structures with even more randomized fiber orientation include the use of felted sheet or mat comprising chopped acid-treated pitch fiber or tow. Felt and mat preforms with volume fractions of from 25 to as great as 80 % may be readily produced, the volume fraction of fiber being selectively determined through control of the felting operation and by use of subsequent compacting process steps. Inasmuch as the fiber alignment in such felted structures will in most instances be random, the thermal and mechanical properties of the resulting self-reinforced carbon article may be nearly or even essentially isotropic.
Although the preform may be carbonized and graphitized without further preparation, thermally processing the wet preform will require evaporation of large quantities of water and it may therefore be desirable to allow excess aqueous composition to fully drain from the fiber, and to carry out the initial low temperature heating steps slowly and in stages to permit some drying of the fiber. It will also be seen to be desirable to provide for removal of the moisture during the low temperature heat stages before finally sealing the carbonizing furnace, in order to reduce the potential for furnace blow-out or other furnace damage due to the presence of large quantities of steam Since the addition of heat cycles increases energy consumption, it may be desirable as an alternative to permit the preform to undergo partial drying at ambient temperatures during the storage period It will be desirable to exercise some care during the drying and storage to ensure that the wound fiber or preform does not sag.
Substantial consolidation occurs during thermal treatment, causing significant change in volume and introducing the possibility for warping and void formation. It will generally be preferred to provide a fixture or mold to control the final shape and permit the application of external pressure where deemed desirable. Generally, the fixture may take a form as complex as the final shape requires, and will be designed to accommodate the volume change of the preform as it becomes consolidated during the carbonizing and graphitizing processes. The production of a simple graphite plate may require no more than sandwiching between rigid, flat sheets, while matched die molds may be necessay for a structure with complex or multiple-curve surfaces. The fixture may be constructed of any material which will withstand the extreme temperatures employed for the thermal treatment without loss of shape or integrity. Generally, graphite will be the material of choice.
Thermal treatment of the preform may be conducted in a single heating step or in stages to a temperature in the range of 1200xc2x0-3500xc2x0 C. to produce carbonized and graphitized carbon articles of this invention. The heat treatment will be conducted in a substantially non-reactive atmosphere to ensure that the fiber is not consumed. The non-reactive atmosphere may be nitrogen, argon or helium; however, for temperatures above about 2000xc2x0 C., argon and helium are preferred. Although the non-reactive atmosphere may include a small amount of oxygen without causing serious harm, particularly if the temperature is not raised too rapidly, the presence of oxygen should be avoided. In addition, wet yarn structures will produce an atmosphere of steam when heated, which should be purged from the furnace before carbonizing temperatures are reached inasmuch as steam is highly reactive at such temperatures. It may be desirable to include boron or similar graphitizing components in the furnace atmosphere and these will be regarded as non-reactive as the term is used herein.
The heat treatment used in carbonizing and graphitizing pitch has three broad ranges which are important in deciding a heating schedule. The rate of temperature increase up to about 400xc2x0 C. should take into account that the pitch fibers will become infusibilized slowly during heating, and may become completely infusibilized when heated above that temperature. Rapid heating may assist softening and fiber deformation due to softening, and cause the fusion and disorientation of the mesophase. While the temperature increase above about 400xc2x0 C. may take place at a higher rate, it must be recognized that much of the gas loss that occurs during the pyrolysis or carbonizing process takes place as the fibers are heated in the range of 400xc2x0 C. to about 800xc2x0 C., and too rapid an increase can result in damage due to evolving gases. Above about 800xc2x0 C., to the final temperature in the range of 1100-2000xc2x0 C. for carbonized structures, and up to 3000xc2x0 and above for graphitizing, the rate of heating may be much greater, and conducted generally at as rapid a rate as may be desired.
A convenient heating schedule includes heating at an initial rate of 20xc2x0 C./hr from room temperature to about 400xc2x0 C., then at 50xc2x0 C./hr from 400xc2x0 to 800xc2x0 C., and finally at a rate of 100xc2x0 C./hr, or even greater if desired, over the range of from about 800xc2x0 C. to the final temperature. The heating schedule also is determined in part upon the type of fiber, the size of the preform, the effective loading of the furnace and similar factors. Various further adjustments may be necessary for use of specific equipment and materials, as will also be readily apparent to those skilled in the art.
It will be understood that although the heat treatment has been described as a single step process, the heating of the preform may in the alternative be conducted in a series of steps or stages, with cooling and storage of intermediate materials such as carbonized structures and preforms for further processing at a later time.
Heat treatment of the acid-treated pitch fiber preform may be carried out either without applying external pressure, or with application of a very low external pressure, preferably from about 0.1 to about 10 psi, to assist the compaction and afford high density composites. Higher pressures, and particularly at the extremely high pressures employed for prior art processes such as those described in U.S. Pat. Nos. 4,350,672 and 4,849,200 for producing reinforced carbon composites, causes the acid-treated fiber of this invention to flow excessively, destroying the orientation needed to provide the final carbon article with good mechanical properties and high thermal conductivity.
It will be readily understood by those skilled in the art that the process of this invention will afford the composite manufacturer a high degree of control over part density and the thermal and mechanical properties in the carbon article. The ability to selectively determine the particular combination of fiber treatment, compaction pressures and thermal processing to be employed with respect to the size and geometry of the part that is being produced will afford an effective means for tailoring properties to particular uses. For large parts, such as those having a thickness of from about 0.25 to about 12 inches, where heat conduction into the center of the part, as well as out-gassing from individual pitch filaments, will necessarily be slow, long heating cycles and slow increases in temperature will be desirable, and higher levels of compacting pressure may be preferred to ensure that a good density will be attained throughout the part. For small parts, particularly those having a thin cross-section of as low as 0.05xe2x80x3, heating may be carried out more rapidly than for larger parts, but the use of lower levels of applied pressure for compacting may be necessary to avoid distorting the part or causing excessive flow within the fiber structure. Where a low preform density is desired, yet a different balance of applied pressures and heating rates will be needed. Balancing the heating rate against the acid treatment parameters affords yet additional degrees of flexibility in the overall process. It will thus be apparent that the manufacturer of carbon articles will be able to select the particular combination of heating parameters and applied pressures, as well as the degree of acid treatment of the fiber, that will determine the properties obtained in the final carbon article.
The carbon articles of this invention may be characterized in terms of their unique combination of physical and mechanical properties. Solid carbon articles prepared according to the invention with a high degree of fiber alignment may have a bulk density higher than is found in most reinforced carbon composites, generally above about 1.8 g/cc, preferably above about 1.9 g/cc, and often approaching that of single crystal carbon. When measured in the direction of the axis of the filamentous domains, the highly dense, self-reinforced carbon articles may exhibit a thermal conductivity greater than 600 w/m-xc2x0K, a tensile strength greater than about 10,000 psi, a tensile modulus above about 70 xc3x97106 psi and a negative coefficient of thermal expansion, as low as about xe2x88x920.5 ppm/xc2x0C.
Although mechanical properties measured in the transverse direction will be considerably lower, articles with transverse tensile strengths greater than 500 psi and a transverse modulus greater than 300,000 psi are readily obtainable by the methods of this invention, while the transverse thermal conductivity will generally be greater than about 40 w/m-xc2x0K and may be as great as about 70 w/m-xc2x0K or more.
As described herein above, control of fiber orientation in the fabrication of the preform may be used to produce carbon articles having lower bulk densities, including high strength, porous carbon articles particularly suited for further processing using infiltration techniques and carbon vapor deposition or infiltration processes to provide unique reinforced carbon structures. These lower bulk density structures will be found on microscopic examination to comprise fully-fused, high-density, highly oriented, high-strength carbon with a high level of open-cell porosity, unlike the low density, reinforced carbon structures of the prior art which are generally made up of carbon fiber, poorly bonded at the points where the fibers are in contact and separated by voids and, where a binder is employed, often including large areas comprising amorphous or low crystallinity carbon. For comparison purposes, commercial high quality bulk graphite materials generally exhibit a much lower bulk density, generally below about 1.6 g/cc, and lower thermal conductivity, ordinarily less than about 185 w/m-xc2x0K. Tensile strengths for such materials are on the order of about 10,000 psi, while the tensile modulus is on the order of about 75,000 psi and the coefficient of thermal expansion is high, generally greater than about +0.7 ppm/xc2x0C. Although highly oriented pyrolytic graphite or HOPG materials may have a bulk density above about 2.0 and a thermal conductivity in the range of 800 w/m-xc2x0K, these unreinforced materials are extremely fragile.
The carbon articles of this invention, whether constructed using a high degree of fiber alignment to be dense or structured to have lower densities, will comprise carbon having an unique morphology with two distinct phases: highly-ordered, large, rod-like crystalline graphite domains, separated and reinforced by highly-ordered, filamentous crystalline graphite. More particularly, it appears that within the solid crystalline carbon comprising the article there is a gradual transition in crystalline form, progressing smoothly from regions comprising large, highly-ordered crystalline graphite domains created from the centers of filament, through regions of highly-ordered filamentous crystalline graphite formed from the oxidized pitch layers that comprised the pitch filament surfaces and extending into the interface regions of filamentous crystalline graphite formed by the knitting together of the most oxidized filamentous mesophase domains located at the contacting filament surfaces.
These smooth transitions in crystal form provide a carbon without grain boundaries and discontinuities such as are characteristically found in prior art reinforced carbon composites, together with a high degree of orientation both in the filamentous graphite and in the rod-like crystal graphite domains. The high degree of physical similarity and chemical identity between components effects a uniform and highly efficient reinforcement of the final graphitic carbon structure, significantly improving toughness. The uniformity of structure and small crystallite morphology, on the order of about 10 microns for the large, highly-ordered crystalline domains, also imparts a high degree of machinability, as well as high tolerance to repetitive temperature cycling.
The preforms are constructed of mesophase pitch fiber. As is known in the art, mesophase or liquid crystal pitch may be readily oriented through use of mechanical operations such as melt-spinning. Such processes are used commercially for spinning filaments comprising continuous, highly-oriented filamentous mesophase pitch domains aligned with the fiber axis. When the oriented liquid crystal pitch is thermally converted into crystalline carbon, the orientation is retained to provide, in the case of fiber, carbon fiber comprising highly oriented, filamentous crystalline carbon.
When treated with liquid oxidizer such as nitric acid, the surfaces of the mesophase pitch filament are altered, providing a filament structure comprising outer layers of oxidized filamentous mesophase domains surrounding a core of substantially unoxidized filamentous mesophase pitch. The level of oxidation and the degree of penetration into the interior of the filament will be determined in part by the oxidizer concentration and the time of exposure, and the filamentous mesophase domains comprising the surfaces of the filament will thus have the highest level of oxidation, while the oxidation levels in the underlying layers of filamentous mesophase will be progressively reduced with distance from the surface. While not intending to be bound by any particular theory of operation, it appears from microscopic examination of the invented carbon articles that during thermal treatment to carbonize and graphitize the preform structure, the pitch filaments initially become deformed radially, flowing to some extent to reduce the void space within the structure and increase the area of contact between filament surfaces. As the pyrolysis, carbonizing and graphitizing of the liquid crystal pitch progresses, the highly-oriented, filamentous mesophase pitch domains comprising the core of the filament appear to undergo some re-crystallization, losing the filamentous character and forming larger crystalline graphite domains while retaining crystal orientation. At the filament surface, the filamentous mesophase domains, oxidized to varying degrees depending on location relative to the surface, appear to form filamentous crystalline carbon domains while becoming more completely adhered or knitted at the contacting surfaces, thus forming a continuous network that extends through and reinforces the carbon structure.
Filled and reinforced composite materials of the prior art, particularly including bulk graphite and carbon fiber-reinforced carbon composites, comprise matrix and discontinuous reinforcement phases embedded in a matrix phase. The phases will differ greatly in crystallinity and often in chemical composition, and thus are highly dissimilar with sharp discontinuities occurring at the phase boundaries as well as at the grain boundaries within their crystalline components. The discontinuities act as flaws, acting to concentrate stress and reduce the strength of the composites and, together with a significant level of amorphous or semi-crystalline character in the matrix component, may further reduce composite density and limit bulk thermal conductivity. In addition, and particularly for prior art composites formed by consolidation of fully oxidized pitch or PAN fiber, the fiber components are adhered with a much lower efficiency and the bonding often fails. Such composites are then difficult to machine, often splintering along fiber interfaces and grain boundaries.